1. Field of the Invention
The present invention relates to a semiconductor memory and a memory system in which internal settings can be changed.
2. Description of the Related Art
Generally, a semiconductor integrated circuit including semiconductor memory conducts a function test upon completion of a product, etc. Therefore, it is necessary to change various internal settings. Furthermore, it is desired for semiconductor memory, etc. that various internal settings such as a BL (burst length) and RL (read latency), etc. depending on the used style and needs.
However, the semiconductor integrated circuit is requested to reduce the number of external terminals with the with recent higher integration and finer circuit patterns of the circuits, there is the tendency that no special external terminals are provided for the input of a signal in conducting a test or setting various operation modes. Therefore, a number of devices are proposed to change various settings without providing exclusive terminals.
For example, a common semiconductor memory is provided with a register for switching operation modes, and the operation mode is changed by externally and electrically rewriting the register information. In many cases, rewriting a register is performed by inputting an illegal command, which is different from normally used commands such as an RD command, a WT command, a Ref command, etc. plural times.
The patent document 1 (Japanese Published Patent Application No. H7-92242) discloses the configuration of assigning a specific address value for mode settings, and writing data to the address value, thereby changing a set value.
As described above, the configuration of changing the settings by enabling a normally illegal command when an operation mode is set, or the configuration of changing the settings of memory by writing data to a specific address value as for the memory described in the patent document 1 are complicated in designing a memory system.